Economic and Environmental Sustainability of Irrigated Grass-Legume Mixtures

View the project final report

• 2016 annual report

Commodities

• Agronomic: general hay and forage crops

Practices

• Education and Training: demonstration, extension, networking, on-farm/ranch research, workshop

Average hay productivity in Wyoming is below the national average. Producers, in an attempt to increase hay production, have resorted to increasing use of commercial nitrogen fertilizers which are costly, pollute underground waters and emit greenhouse gas, such as nitrous oxide. The increasing reliance on fertilizers led to a surge in dollars spent on nitrogen fertilizers by producers within the past few years. This has not only increased the cost of production and reduced farm profits but also forced most producers out of business. The number of acres of land under hay production in Wyoming reduced significantly between 2011 and 2012. Though grass-legume mixtures offer a potential for addressing these challenges, poor persistence of legumes in grass-legume mixtures discourages producers from establishing mixed stands. Growing incompatible grass-legume species also reduces biological nitrogen fixation and forage yield and quality. Aside from the lack of region-specific information on grass-legume mixtures in Wyoming, previous research focused on single ratio of grass-legume mixtures and failed to access the impact on producer farm profits. Identifying grass-legume species which are compatible and increase nitrogen fixation, forage yield and quality and overall farm profit will be important to Wyoming producers. The goal of the project is to identify how different grass-legume mixture ratios will use soil moisture and nutrients and affect stand life, productivity and the profitability of such a system. This study fits into the mission of Western SARE “…to advance-to the whole of American Agriculture-innovations that improve profitability…” A field study will be conducted to assess benefit-cost ratios of using grass-legumes mixtures and sole grass (with and without nitrogen) and legume stands. Yield, quality and efficiency of nitrogen and water use will also be measured. Data obtained from this study will form the main component of the student's PhD dissertation. The strength of this study lies in the focus on the overall sustainability of the farm systems. This research will not only address the issue of dwindling hay production and farm profits in Wyoming, but also how nitrogen fertilizer rates affect soil conditions and soil microbial structure. Using different ratios of two or three legumes in mixtures with a grass species will help identify which combinations of legume and grass species are compatible. Inclusion of two or more legume species in mixtures with a grass species means if one legume species “disappears,” the other legume species could still remain, which will prolong the stand life and, hence, farm yield and profitability. When compatible legume and grass species are grown, nitrogen fixation is enhanced, mitigating the need for nitrogen fertilization. Identifying nitrogen rates that produce high yield and quality forage and reduce excessive nitrogen use will be essential in reducing groundwater pollution and greenhouse gas emission.

The main goal of this study is to identify grass-legume mixture ratios that will produce higher yields of good quality forage and be efficient in the use of nitrogen fertilizer and water and profitable to Wyoming producers. An additional goal is to assess the production system's effects on soil microorganisms, pH and soil organic matter.

Specific objectives of the study include:

1. Assess the effects of different ratios of grass-legume mixtures and nitrogen fertilizer rates on forage yield and quality;
2. Determine the effects of grass-legume mixtures on the persistence of legumes;
3. Assess the efficiency of water and nitrogen fertilizer use in the grass-legume mixtures cropping systems;
4. Determine the economic profitability of sole grass stands fertilized with nitrogen, sole legume stands and grass-legume mixtures;
5. Evaluate the effects of grass-legume mixtures on soil organic matter buildup, pH and microorganism populations.

Each objective will be accomplished by a series of studies/methods.

Objective 1: Forage yield in each system will be calculated based on the dry matter per unit sampled area. Forage quality will be defined based on crude protein (CP), in-vitro dry matter digestibility (IVDMD), acid detergent fiber (ADF), neutral detergent fiber (NDF) and total digestible nutrients (TDN) contents.

Objective 2: Persistence of legumes in grass-legume mixtures will be determined by botanical composition method and non-structural carbohydrate content of roots. By botanical composition, weight decline in legume component relative to total plot biomass in each system will be used.

Objective 3: Amount of nitrogen extracted by plants relative to residual soil nitrogen and total nitrogen applied will be measured. These will be used to calculate nitrogen recovery index (NRI; fertilizer not used by plants) and fertilizer nitrogen use efficiency (FNUE). Finally, WUE will be quantified.

Objective 4: Profitability of each system will be evaluated based on the gross revenue relative to total cost of establishment and management.

Objective 5: Soil organic matter, pH and microorganism population structure will be determined at the end of the study and compared to the pre-planting analyses results. This will help determine the overall effect of each system on agricultural soil.